1. Field of the Invention
The present invention relates to a gradation correction device for correcting gradation of input picture signals and a device therewith such as a camcorder or a video printer.
2. Description of the Prior Art
When a dark object is taken in a picture with a camcorder or the like, the object in an output picture becomes darkened with poor gradation. In particular, when a back-lit object is taken in a picture, extreme black or white portions appear in the picture and an output picture becomes unnatural. Therefore, a gradation correction device is needed to be installed in a camcorder or the like which processes picture signals of a 1-field picture.
Many gradation correction circuits have been developed. For example, a prior art gradation correction circuit is disclosed in Japanese Laid-open Patent Publication No. 125,589/1990 on the correction of image quality of an image sensing device. In the gradation correction circuit, a luminance signal Y is first attenuated and next amplified nonlinearly. A nonlinear portion is extracted from amplified signal and added to the luminance signal Y. Thus, half-tone signals are emphasized while signals of high and low luminances are preserved. Thus, the image quality of a main object and background under back light can be improved.
However, in the above-mentioned prior art, only intermediate luminance levels are expanded. Therefore, it is a problem for a strongly back-lit object that gradation becomes poor between intermediate and high levels of picture signals. Further, though image quality can be increased a little for an object under back light, gradation for a front-lit object or for am object having gradation ranging from low to high levels becomes poor in some portions or an output image becomes unnatural.
A second prior art gradation correction circuit is disclosed in Japanese Laid-open Patent Publication No. 206282/1990. In the above-mentioned circuit, image signals are divided into predetermined ranges of signal levels, and an average or an integrated value of image signals in each level range is detected. A gain control data ROM has stored a gain or attenuation amount for a gamma correction controller in correspondence with the average of image signals in each level range. By controlling the gamma correction characteristic in correspondence with he output signal of the gain control data ROM, extreme white or black portions can be prevented from appearing in an output picture.
In the above-mentioned second prior art, the deterioration of an image due to extreme black portions can be prevented by increasing the gain at low luminance, while the deterioration of an image at extreme white portions can be prevented by controlling the attenuation at intermediate and high luminances. However, it is a problem that gradation at high luminance becomes poor. Further, though gradation correction characteristic has to be changed for front-lit objects and for back-lit objects, it is not easy to change gradation correction characteristic for front-lit objects and for back-lit objects. It is also a problem that an output picture becomes unstable and unnatural if the prior art gradation correction circuit is applied to a video camera which deals a moving picture.